1. Field of the Invention
The invention relates to a method for thermally processing polyester pellets in order to achieve crystallization.
2. Discussion of Background Information
Polyethylene terephthalate, hereinafter also referred to herein as PET for short, is a polyester with repeating ester groups.
PET can be present in different structures, namely in amorphous or in crystalline or partially crystalline form. Amorphous PET is mostly transparent, and crystalline PET is opaque or white. As with all thermoplastics that can be present in amorphous or crystalline form, a 100% degree of crystallization is also not possible with PET. Only a portion of the structure of the PET is able to orient itself, i.e., to crystallize, and PET includes crystalline and amorphous regions which alternate. Therefore, PET is always referred to with respect to partial crystallinity. An approximate 50% degree of crystallinity can be achieved with PET in order to prevent the pellets or granules from sticking to one another. This means that in this state about half of the molecule chains have oriented themselves to one another, and thus have laid themselves parallel next to one another or have wound themselves in a circular manner. The interactions (van der Waals forces) between the molecular chains therefore inevitably become greater in the partially crystalline regions. The chains thus mutually attract one another and thus gaps between the molecules become smaller.
As a thermoplastic, PET can be molded at temperatures of 250° C. The molecular chains then become so mobile that the plastic melts and a viscous mass results that can be made into virtually any desired shape. When it cools, the molecular chains refreeze, and the plastic solidifies in the desired shape—a simple principle that can be repeated multiple times. This method is also used, e.g., in the production of PET bottles. So-called preforms are produced in a first step. As a precursor of the PET bottles, these preforms already have a finished screw thread. In order to obtain proper bottles, they are again softened at 100° C., stretched with compressed air and blown to produce a bottle (stretch blow process).
The production of crystallized PET in pellet form hitherto comprised extensive and complicated fluidized bed methods that required large investments and high operating costs, such as DE 198 48 245 A and its family member WO 00/23497, which are incorporated by reference herein in their entireties.
PET pellets have to be crystallized at a temperature below that at which the material becomes sticky in order to prevent the pellets coalescing into a solid mass that can barely be processed. Although the melting temperature of the crystallized polyester is not reached until 240 to 250° C., it can already become sticky before crystallization at temperatures above approx. 70° C.
In so far as continuous methods for producing dry PET pellets are known, in general they require very large installations, since long crystallization times are necessary.
Thus, for example, U.S. Pat. No. 5,532,335, which is incorporated by reference herein in its entirety, is directed to a method for thermally processing polyester pellets in which the pellets are introduced into a processing vessel and a liquid medium is also introduced into this processing vessel, whereby the pellets and the liquid medium are mixed together. Pressurized water or so-called superheated water is used hereby as a liquid medium in the proposed process. The boiling temperature can easily be controlled by changing the pressure in the reactor vessel. In an exemplary embodiment polyester pellets are processed at 120°-182° C. The water is introduced at 160° C., kept in the liquid state and the pellets are added as long as the pressure in the reactor unit is kept at 7 kg/cm2 or higher. It is evident that such a method is extraordinarily expensive and therefore can barely be conducted economically.
Known methods that work with an aerodynamic processing also have the serious disadvantage that they use a large amount of inert gases. The energy and processing costs are also too high here for a practical large-scale application.
In order to sufficiently crystallize the material in the prior art it was therefore always necessary to add sufficient external energy or heat to the crystallization process. These cited problems have hitherto hampered PET recycling.